In fact, the overall perspective and conception of a synthesis commences with a careful logical dissection of the target-drug-molecular skeleton into synthons. However, the disconnection of a bond within a monocyclic system shall be a retro-synthetic ring-opening phenomenon, otherwise termed as the retro-synthetic approach. Likewise, the disconnection of a bond caused in a bridged-structure would ultimately produce either a mono- or a di- substituted monocyclic structure. Sometimes, it may also be possible to accomplish two-bond disconnections taking place almost simultaneously.
A double-line arrow is invariably used to indicate a reaction written backwards—the actual reaction in reverse. The retro-synthetic approach may be expaliated with the help of the following classical example of vitamin A :
Potential Synthon Skeletons
A good, knowledgeable and academically competent research chemist is fully aware of the host of organic chemical reactions that are implied either directly or indirectly in designing synthesis. It is fairly understood and appreciated that common organic compound(s) and reagent(s) must be sourced through genuine and well-reputed manufactures round the world whose products are not only authentic but also cent-per-cent reliable and trustworthy, namely : Aldrich, Sigma, Fluka, BDH, Merck, Qualigens, Loba, and the like. Paradoxically, one may expect a, pure and reasonably good desired 'target-drug-molecule' if and only when one makes use of pure starting materials ; of course, under rigid experimental conditions.
Broadly speaking, a research chemist profusely utilizes his wisdom and skill in designing the synthesis of rather complex natural products by employing relatively small synthons comprising of 1 to 5 C-atoms. In the event, when the target-drug-molecule contains a benzene ring, the selection of an aromatic starting material is invariably the best choice. In fact, the genuine demand of a chemist to have a relatively large starting-material molecule is fairly justified so as to minimise and cut-short the number of essential construction-reactions ; but unfortunately the quantum of such molecules are absolutely rare and scarce. It has also been a regular practice in designing a synthesis to make use of either naturally occurring starting material or an already synthesized chemical entity.
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